i've had the same annoyance. The means I use to achieve this is via the "custom property editor" on the console/server. Select "Nodes" table. Then select "No Active Filter" build filter - to select ALL nodes that match (ie: Records where CITY is empty) would pick up all nodes with Unknown city. Once results are returned, you can sort by selecting column, ie: Site or IP_Address etc. Or build a more complex filter. If you have lots of updates to perform, we typically export/import, using excel to edit/populate custom fields.